X-ray attenuates while it passes through a substance. The attenuation depends on an atomic number of an element constituting the substance and density and thickness of the substance. A probe for examining the inside of an object using X-ray exploits this X-ray attenuation property. X-ray imaging is commonly used in medical diagnoses and non-destructive inspections.
A common X-ray imaging system captures a radiograph or X-ray transmission image of an object arranged between an X-ray source for applying X-ray and an X-ray image detector for detecting the X-ray. The X-ray emitted from the X-ray source attenuates or is absorbed by the object depending on the object's properties (atomic number, density, thickness) while the X-ray passes through the object, and then enters each pixel in the X-ray image detector. Thereby, the X-ray image detector detects and produces an X-ray absorption image of the object. A flat panel detector (FPD), photostimulable phosphor, and a combination of an intensifying screen and a film are commonly used as the X-ray image detectors.
The X-ray absorption property of a substance decreases as the atomic number of the element constituting the substance decreases. This causes a problem that a sufficient contrast cannot be obtained in the X-ray absorption image of living soft tissue or soft materials. For example, cartilage in a joint of a human body and synovial fluid surrounding the cartilage are mainly made of water, so there is little difference between their amounts of X-ray absorption, resulting in little difference in contrast.
Recently, X-ray phase imaging has been studied actively to solve the above problem. The X-ray phase imaging obtains an image (hereafter referred to as phase contrast image) based on phase changes (angle changes), instead of intensity changes, of the X-ray caused by the object through which the X-ray passes. Generally, when the X-ray is incident on the object, the object interacts with the phase of the X-ray more strongly than with the intensity of the X-ray. Accordingly, the X-ray phase imaging using the phase difference obtains a high contrast image even if the object is composed of components with little difference in their X-ray absorptivity. Recently, an X-ray imaging system using an X-ray Talbot interferometer is devised as an example of the X-ray phase imaging. The X-ray Talbot interferometer is composed of an X-ray source, two transmission diffraction gratings, and an X-ray image detector (see, for example, Japanese Patent Laid-Open Publication No. 2008-200359, and C. David, et al., “Differential X-ray Phase contrast imaging using a shearing interferometer”, Applied Physics Letters, Vol. 81, No. 17, October 2002, page 3287).
In an X-ray Talbot interferometer, an object is arranged between an X-ray source and a first diffraction grating. A second diffraction grating is arranged downstream of the first diffraction grating by the Talbot length defined by the grating pitch of the first diffraction grating and the X-ray wavelength. The X-ray image detector is arranged behind the second diffraction grating. A Talbot length is a distance between the first diffraction grating and a position at which the X-ray passed through the first diffraction grating forms a self image of the first diffraction grating due to the Talbot effect. The self image is modulated due to the interaction between the X-ray and the object arranged between the X-ray source and the first diffraction grating, namely, the interaction changes the phase of the X-ray.
The X-ray Talbot interferometer detects moiré fringes generated by superposition (intensity modulation) of the self image of the first diffraction grating and the second diffraction grating using a fringe-scanning method. Then the X-ray Talbot interferometer obtains a phase contrast image of the object H from changes in the moiré fringes caused by the object H. In the fringe-scanning method, images are captured while the second diffraction grating is translationally moved in a direction substantially parallel to the plane of the first diffraction grating and substantially vertical to the grating direction of the first diffraction grating at a scanning pitch which is one of equally-divided parts of a grating pitch, and then a phase differential image is obtained from a phase shift value of the intensity changes in the pixel data, obtained by each pixel in the X-ray image detector, caused by the scanning. The phase shift value is a value of the phase shift between the case where the object H is present and the case where the object H is absent. The phase differential image corresponds to angular distribution of the X-ray refracted by the object. The phase differential image is integrated in the fringe-scanning direction. Thereby, a phase contrast image of the object is obtained. Because the pixel data is a signal whose intensity is periodically modulated by the scanning, a set of pixel data obtained by the scanning is referred to as an intensity modulated signal. An imaging apparatus using laser light instead of X-ray also employs the fringe-scanning method (for example, see Hector Canabal, et al., “Improved phase-shifting method for automatic processing of moiré deflectograms” Applied Optics, Vol. 37, No. 26, September 1998, page 6227).
The X-ray imaging system employing the X-ray Talbot interferometer uses a solid-state imaging device, for example, the above-described FPD, which obtains pixel data as digital data, as the X-ray image detector. Such X-ray image detector is provided with a plurality of pixels. Inevitably, defective pixels occur. Here, the term “defective pixel” includes a physically defective pixel caused by production and the like and a pixel outputting an abnormal or unexpected signal value due to various reasons such as a flaw on a detection surface or a deposit of foreign matter although the pixel functions normally.
To correct the defective pixels, a technique to obtain positional information of the defective pixels in advance to perform correction processing to an X-ray imaging obtained by an X-ray image detector is known (see, for example, Japanese Patent Laid-Open Publication No. 2008-079923 and Japanese Patent Laid-Open Publication No. 2002-197450).
In the above-described X-ray imaging system, the first and second diffraction gratings are required to be produced with high precision and a small production error. Even a slight deformation in one of the first and second diffraction gratings causes a pixel in a position corresponding to the deformation to fail to detect a normal phase shift value. Such pixel functions as a defective pixel. A method for detecting defective pixels caused by the grating deformation has not been known.
Specific examples of the deformation of the first and second diffraction gratings include an irregular grating pitch, an irregular opening width, thickness unevenness of the grating, local inclination of the grating, and the like. Such deformation affects not only the X-ray transmittance but also the intensity modulated signal obtained with the positions of the first and second diffraction gratings relatively shifted. As a result, the detection accuracy of the phase shift value is degraded, making it difficult to detect defective pixels caused by the grating deformation based on the absorption image. This absorption image is obtained by X-ray imaging using the first and second diffraction gratings. The above-described Japanese Patent Laid-Open Publication No. 2008-079923 and Japanese Patent Laid-Open Publication No. 2002-197450 are not related to the X-ray phase imaging and do not touch upon the method for detecting the defective pixel caused by the grating deformation.
To obtain a phase contrast image from a phase differential image, the above-described X-ray imaging system requires an integration process in the fringe-scanning direction. One defective pixel results in linear artifact in the fringe-scanning direction, so it is desired to detect the defective pixel with high accuracy.